Other mistake: Two cars with the same license plate numbers are seen as the minions are hitch hiking on the road. The plates of the hippie wagon and the old lady's car both read D-6413.
Other mistake: In the end credits, the additional voices are listed alphabetically by last name (column form); however Carlos Alazraqui is listed before Lori Alan.
Chosen answer: The writers wanted to keep Gru's cameo at the end of the film a secret for the fans of the franchise. So, saying "42 Years B.G" (before Gru) in the trailer doesn't give the surprise away.